Does decrease in ribulose-1,5-bisphosphate carboxylase by antisense rbcS lead to a higher N-use efficiency of photosynthesis under conditions of saturating CO₂ and light in rice plants?

Rice (Oryza sativa L.) plants with decreased ribulose-1,5-bisphosphate carboxylase (Rubisco) were obtained by transformation with the rice rbcS antisense gene under the control of the rice rbcS promoter. The primary transformants were screened for the Rubisco to leaf N ratio, and the transformant with 65% wild-type Rubisco was selected as a plant set with optimal Rubisco content at saturating CO₂ partial pressures for photosynthesis under conditions of high irradiance and 25°C. This optimal Rubisco content was estimated from the amounts and kinetic constants of Rubisco and the gas-exchange data. The R₁ selfed progeny of the selected transformant were grown hydroponically with different N concentrations. Rubisco content in the R₁ population was distributed into two groups: 56 plants had about 65% wild-type Rubisco, whereas 23 plants were very similar to the wild type. Although the plants with decreased Rubisco showed 20% lower rates of light-saturated photosynthesis in normal air (36 Pa CO₂), they had 5 to 15% higher rates of photosynthesis in elevated partial pressures of CO₂ (100-115 Pa CO₂) than the wild-type plants for a given leaf N content. We conclude that the rice plants with 65% wild-type Rubisco show a higher N-use efficiency of photosynthesis under conditions of saturating CO₂ and high irradiance.